From my personal experience with average Joe, forcing people to deal with extension might not work.

The idea behind extensions is that user's don't have to deal with them, the computer does the heavy lifting. If Adobe ePubs were .epuba then FBReader (say) would not even try to open them, and Adobe Digital Editions would open them by default. Similarly, .epube would be mapped to eReader and would be ignored by the other Readers. However, Adobe has already "squatted" on .epub and are unlikely to change to .epuba. I have a similar post in the .ePUB extension should be reserved for non-DRM eBooks thread, which is a better location for discussion of how to implement this.